The primary goals of this proposal are: (a) To investigate the genetic structure of the putative mer operons that contain the genes governing resistance to inorganic Hg2 ion and to organomercurials; (b) To characterize the enzymes and other proteins required for degradation of organomercurials and reduction of inorganic mercury; and (c) To study the relationship between these systems in different groups of bacteria. In all known cases, mercury and organomercurial resistance is due to the inducibly regulated synthesis of enzymes that degrade such mercurials as phenylmercury and methylmercury to benzene and methane plus Hg2 ion; and that then reduce Hg ion and Hg2 ion to volatile Hg. The genes governing these resistances have been found invariably on resistance plasmids and not on bacterial chromosomes. In addition to our studies of mercury resistance, we will address two different cases of plasmid-determined resistances to heavy minerals, that of cadmium resistance determined by S. aureus plasmids and of arsenate resistance determined by E. coli plasmids. In both cases the operational hypothesis is of altered active transport systems with diminished affinities for the toxic analogs cadmium or arsenate relative to the "natural" substrates manganese and phosphate.